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  • richardmitnick 11:36 am on October 19, 2020 Permalink | Reply
    Tags: "The Milky Way galaxy has a clumpy halo", , , , CGM - CircumGalactic Medium, , HaloSat minisatellite, It seems as if the Milky Way and other galaxies are not closed systems., So it turns out with HaloSat alone we really can’t say whether or not there really is this extended halo., The next step is to combine the HaloSat data with data from other X-ray observatories ., There still could be a really big extended halo that is just dim in X-rays.., , What we’ve done is definitely show that there’s a high-density part of the CGM that’s bright in X-rays.   

    From University of Iowa: “The Milky Way galaxy has a clumpy halo” 

    From University of Iowa

    Astronomers at the University of Iowa have determined our galaxy is surrounded by a clumpy halo of hot gases that is continually being supplied with material ejected by birthing or dying stars. The halo also may be where matter unaccounted for since the birth of the universe may reside. Credit: Christien Nielsen/Unsplash.

    Richard C. Lewis
    Office of Strategic Communication

    The Milky Way galaxy is in the recycling business.

    University of Iowa astronomers have determined our galaxy is surrounded by a clumpy halo of hot gases that is continually being supplied with material ejected by birthing or dying stars. This heated halo, called the circumgalactic medium (CGM), was the incubator for the Milky Way’s formation some 10 billion years ago and could be where basic matter unaccounted for since the birth of the universe may reside.

    The findings come from observations made by HaloSat, one of a class of minisatellites designed and built at Iowa—this one primed to look at the X-rays emitted by the CGM.

    HaloSat- Credit Blue Canyon Technologies.

    The researchers conclude the CGM has a disk-like geometry, based on the intensity of X-ray emissions coming from it. The HaloSat minisatellite was launched from the International Space Station in May 2018 and is the first minisatellite funded by NASA’s Astrophysics Division.

    “Where the Milky Way is forming stars more vigorously, there are more X-ray emissions from the circumgalactic medium,” says Philip Kaaret, professor in the Iowa Department of Physics and Astronomy and corresponding author on the study, published online in the journal Nature Astronomy. “That suggests the circumgalactic medium is related to star formation, and it is likely we are seeing gas that previously fell into the Milky Way, helped make stars, and now is being recycled into the circumgalactic medium.”

    Each galaxy has a CGM, and these regions are crucial to understanding not only how galaxies formed and evolved but also how the universe progressed from a kernel of helium and hydrogen to a cosmological expanse teeming with stars, planets, comets, and all other sorts of celestial constituents.

    HaloSat was launched into space in 2018 to search for atomic remnants called baryonic matter believed to be missing since the universe’s birth nearly 14 billion years ago. The satellite has been observing the Milky Way’s CGM for evidence the leftover baryonic matter may reside there.

    To do that, Kaaret and his team wanted to get a better handle on the CGM’s configuration.

    More specifically, the researchers wanted to find out if the CGM is a huge, extended halo that is many times the size of our galaxy—in which case, it could house the total number of atoms to solve the missing baryon question. But if the CGM is mostly comprised of recycled material, it would be a relatively thin, puffy layer of gas and an unlikely host of the missing baryonic matter.

    “What we’ve done is definitely show that there’s a high-density part of the CGM that’s bright in X-rays, that makes lots of X-ray emissions,” Kaaret says. “But there still could be a really big, extended halo that is just dim in X-rays. And it might be harder to see that dim, extended halo because there’s this bright emission disc in the way.

    “So it turns out with HaloSat alone, we really can’t say whether or not there really is this extended halo.”

    Kaaret says he was surprised by the CGM’s clumpiness, expecting its geometry to be more uniform. The denser areas are regions where stars are forming, and where material is being traded between the Milky Way and the CGM.

    “It seems as if the Milky Way and other galaxies are not closed systems,” Kaaret says. “They’re actually interacting, throwing material out to the CGM and bringing back material as well.”

    The next step is to combine the HaloSat data with data from other X-ray observatories to determine whether there’s an extended halo surrounding the Milky Way, and if it’s there, to calculate its size. That, in turn, could solve the missing baryon puzzle.

    “Those missing baryons better be somewhere,” Kaaret says. “They’re in halos around individual galaxies like our Milky Way or they’re located in filaments that stretch between galaxies.”

    Study co-authors include Jesse Bluem, graduate student in physics at Iowa; Hannah Gulick, graduate student in astronomy at the University of California, Berkeley who graduated from Iowa last May; Daniel LaRocca, who earned his doctorate at Iowa last July and is now a postdoctoral researcher at Pennsylvania State University; Rebecca Ringuette, a postdoctoral researcher with Kaaret who joined NASA’s Goddard Space Flight Center this month; and Anna Zayczyk, a former postdoctoral researcher with Kaaret and a research scientist at both NASA Goddard and University of Maryland, Baltimore County.

    NASA funded the research.

    See the full article here.


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    The University of Iowa is a public research university in Iowa City, Iowa. Founded in 1847, it is the oldest and the second-largest university in the state. The University of Iowa is organized into 12 colleges offering more than 200 areas of study and seven professional degrees.

    On an urban 1,880-acre campus on the banks of the Iowa River, the University of Iowa is classified among “R1: Doctoral Universities – Very high research activity”. The university is best known for its programs in health care, law, and the fine arts, with programs ranking among the top 25 nationally in those areas. The university was the original developer of the Master of Fine Arts degree and it operates the Iowa Writer’s Workshop, which has produced 17 of the university’s 46 Pulitzer Prize winners. Iowa is a member of the Association of American Universities, the Universities Research Association, and the Big Ten Academic Alliance.

    Among American universities, the University of Iowa was the first public university to open as coeducational, opened the first coeducational medical school, and opened the first Department of Religious Studies at a public university. The University of Iowa’s 33,000 students take part in nearly 500 student organizations. Iowa’s 22 varsity athletic teams, the Iowa Hawkeyes, compete in Division I of the NCAA and are members of the Big Ten Conference. The University of Iowa alumni network exceeds 250,000 graduates.

  • richardmitnick 9:17 am on August 25, 2020 Permalink | Reply
    Tags: "Will Radio Bursts Reveal Hidden Baryons?", , , , , CGM - CircumGalactic Medium, , , , ,   

    From AAS NOVA: “Will Radio Bursts Reveal Hidden Baryons?” 


    From AAS NOVA

    24 August 2020
    Susanna Kohler

    Australian Square Kilometre Array Pathfinder (ASKAP) is a radio telescope array located at Murchison Radio-astronomy Observatory (MRO) in the Australian Mid West. ASKAP consists of 36 identical parabolic antennas, each 12 metres in diameter, working together as a single instrument with a total collecting area of approximately 4,000 square metres.

    Transients like fast radio bursts, detected with telescopes like the ASKAP array may be the key to identifying how much matter is hiding in our galaxy’s diffuse halo.

    The Earth, your body, and the electronic device you’re reading this on are all made up of ordinary, baryonic matter. A new study has now used bursts of radio emission to probe whether the outskirts of our galaxy are hiding vast quantities of “missing” baryonic matter.

    Missing Matter

    The relative amounts of the different constituents of the universe. Ordinary baryonic matter makes up less than 5%. [ESA/Planck.]

    We’ve long known that only about 5% of the content of the universe is ordinary baryonic matter; the remainder is dark matter and dark energy. But when scientists have searched for this baryonic matter in the nearby universe, they found a puzzle: galaxies’ gas, dust, and stars only accounted for a small fraction of their expected baryonic matter.

    Our own Milky Way is no exception — it also has a baryon fraction much lower than the overall baryon fraction in the universe. So where are its missing baryons? Were they expelled from our galaxy at some point in the past? Or did the Milky Way retain its baryons — but we haven’t detected them yet?

    An Elusive Halo

    If our galaxy’s baryons are still around, a likely hiding place is in the Milky Way’s outskirts, in the circumgalactic medium (CGM).

    The Sombrero galaxy, M104, provides an example of a galaxy and its halo — the diffuse gas that extends above and below the galaxy’s disk. [ESA/C. Carreau.]

    When our galaxy formed, gas was dragged inward with the collapsing dark-matter halo, shock heating and forming a surrounding bubble of hot, diffuse plasma — the CGM. This surrounding galactic halo may well contain our galaxy’s missing baryons today, but it’s very difficult to probe; since the gas is diffuse, we can’t measure it directly from within the Milky Way.

    A new study led by Emma Platts (University of Cape Town, South Africa) has instead measured the galactic halo’s matter by observing how distant signals interact with the CGM as they travel to us.

    Clues from Transients

    Platts and collaborators use two types of radio transients to measure CGM distribution: pulsars, which are pulsating neutron stars that reside in our galaxy’s disk, and fast radio bursts, which are brief flashes of radio emission that originate far beyond our galaxy.

    Pulsars, which typically lie in the galactic disk, emit radiation that sweeps over the Earth like a lighthouse, appearing as pulses. These pulses become dispersed as they travel through the galaxy to reach us. [Bill Saxton/NRAO/AUI/NSF]

    Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.

    Light from these sources travels across space to us, interacting with matter distributed along the way. The interactions slow down longer wavelengths of light more than shorter, causing the signal to spread out. The dispersion measure — the quantification of this spread — therefore tells us how much matter the signal traveled through to get to us.

    Probing Our Surroundings

    By statistically analyzing the distribution of pulsar and fast radio burst dispersion measures, Platts and collaborators placed bounds on the Milky Way halo’s dispersion measure: its minimum is set by the farthest pulsars, which lie interior to the halo, and its maximum is set by the closest fast radio bursts, which lie far beyond our halo in neighboring galaxies.

    Milky Way Halo NASA/ESA STScI

    So are the Milky Way’s missing baryons hiding in the CGM? We can’t say for certain yet, but the results suggest no, if the baryons are distributed in the same way as the dark matter. The future should hold more certainty though! Our sample of fast radio bursts is rapidly growing, and the authors estimate that once we’ve cataloged several thousand, we’ll be able to bound the content of the Milky Way’s halo more definitively.

    Schematic illustrating how transient radio signals travel to us. Pulsars (marked by sun symbols) lie in the galaxy, interior to the halo; their signals are dispersed only by the Milky Way’s interstellar matter. Fast radio bursts (marked by lightning symbol) lie in other galaxies; their signals are dispersed by the Milky Way’s interstellar matter, its halo, the intergalactic medium, the host galaxy’s halo, and the host itself. These two types of transients can therefore place upper and lower bounds on the matter in the Milky Way’s halo. [Platts et al. 2020]


    “A Data-driven Technique Using Millisecond Transients to Measure the Milky Way Halo,” E. Platts et al 2020 ApJL 895 L49.


    See the full article here .


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    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

  • richardmitnick 10:45 am on December 6, 2018 Permalink | Reply
    Tags: , , , CGM - CircumGalactic Medium, , , , NASA/ESA Cosmic Origins Spectrograph, , The ecosystem that controls a galaxy’s future is coming into focus   

    From Science News: “The ecosystem that controls a galaxy’s future is coming into focus” 

    From Science News

    July 12, 2018
    Lisa Grossman

    The circumgalactic medium has been hard to observe, but new tools now make it possible.

    COSMIC CLOAK Whirls of cold and hot gas billow in this simulation of a circumgalactic medium surrounding a galaxy. With new tools and simulations, researchers have learned that the CGM helps a galaxy recycle its materials. M.S. Peeples et al/FOGGIE Project

    There’s more to a galaxy than meets the eye. Galaxies’ bright stars seem to spiral serenely against the dark backdrop of space. But a more careful look reveals a whole lot of mayhem.

    “Galaxies are just like you and me,” Jessica Werk, an astronomer at the University of Washington in Seattle, said in January at a meeting of the American Astronomical Society. “They live their lives in a constant state of turmoil.”

    Much of that turmoil takes place in a huge, complicated setting called the circumgalactic medium, or CGM. This vast, roiling cloud of dust and gas is a galaxy’s fuel source, waste dump and recycling center all in one [Annual Review of Astronomy and Astrophysics]. Astronomers think the answers to some of the most pressing galactic mysteries — how galaxies keep forming new stars for billions of years, why star formation abruptly stops — are hidden in a galaxy’s enveloping CGM.

    “To understand the galaxies, you have to understand the ecosystem that they’re in,” says astronomer Molly Peeples of the Space Telescope Science Institute in Baltimore.

    Yet this galactic atmosphere is so diffuse that it’s invisible — a liter of CGM contains just a single atom. It has taken almost 60 years and an upgrade to the Hubble Space Telescope just to begin probing distant CGMs and figuring out how their constant churning can make or break galaxies.

    “Only recently have we been able to really, truly, observationally characterize the relationship between this gaseous cycle and the properties of the galaxy itself,” Werk says.

    Armed with the first extragalactic census, astronomers are now piecing together how a CGM controls its galaxy’s life and death. And new theoretical studies hint that galaxies’ stars would be arranged very differently without a medium’s frenetic flows. Plus, new observations show that some CGMs are surprisingly lumpy [Nature]. A better understanding of CGMs, enabled by new telescopes and computer simulations, could change how scientists think about everything from galaxy collisions to the origins of our own atoms.

    “The CGM is the part of the iceberg that’s under the water,” says astrophysicist Kevin Schawinski of ETH Zurich, who studies the more conventional parts of galaxies. “We now have good measurements where we’re sure it’s important.”

    Frenetic fog

    Researchers use a bright source of background light, like a quasar, to learn about a galaxy’s circumgalactic medium, a diffuse cloud of gas and metals (pink in the illustration) surrounding a galaxy. Gas is recycled between the galaxy and the CGM.

    Sources: J. Tumlinson, M.S. Peeples and J.K. Werk/Annual Review of Astronomy and Astrophysics 2017; M.S. Peeples/Nature 2015

    Waiting for Hubble

    That 2009 Hubble telescope upgrade, which made the CGM census possible, almost didn’t happen.

    In a cosmic coincidence, the Hubble telescope’s chief champions were also the first astronomers to figure out how to observe a galaxy’s CGM. Lyman Spitzer of Princeton University and John Bahcall of the Institute for Advanced Study in Princeton, N.J., and other astronomers noticed something strange after the 1963 discovery of quasars [http://cosmology.carnegiescience.edu/timeline/1963] (SN Online: 3/21/14), bright beacons now known to be white-hot disks surrounding supermassive black holes in the centers of distant galaxies.

    Everywhere astronomers looked, quasars’ spectra — the rainbow created when their light is spread out over all wavelengths — were notched with dark holes. Some wavelengths of light weren’t getting through.

    In 1969, Spitzer and Bahcall realized what was going on: The missing light was absorbed by gas at the edges of galaxies, the same stuff that would later be called the CGM. Astronomers had been peering at quasars shining through CGMs like headlights through a fog.

    Not much more could be done at the time, though. Earth’s atmosphere also absorbs light in those same wavelengths, making it difficult to tell which light-blocking atoms were in a galaxy’s CGM and which came from closer to home. Knowing that a CGM was there was one thing; taking its measurements would require something extra.

    Spitzer and Bahcall knew what they needed: a space telescope that could observe from outside Earth’s atmosphere. The pair were two of the most vocal and consistent champions of the Hubble Space Telescope, which launched in 1990. Spitzer’s colleagues called him Hubble’s “intellectual and political father.”

    Bahcall never stopped advocating for Hubble. In February 2005, six months before his death at age 70 from a rare blood disorder, he co-wrote an article in the Los Angeles Times [http://articles.latimes.com/2005/feb/23/opinion/oe-tayloretal23] urging Congress to restore funding for a mission to fix some aging Hubble instruments, which NASA had canceled after the 2003 Columbia space shuttle disaster.

    “What is at stake is not only a piece of stellar technology but our commitment to the most fundamental human quest: understanding the cosmos,” Bahcall and colleagues wrote. “Hubble’s most important discoveries could be in the future.”

    His plea was answered: The space shuttle Atlantis brought astronauts to repair Hubble for the last time in May 2009 (SN Online: 5/19/09). During the repair, the astronauts installed the Cosmic Origins Spectrograph, which could pick up diffuse CGM gas with 30 times the sensitivity of any previous instrument.

    NASA Hubble Cosmic Origins Spectrograph

    Although earlier spectrographs on Hubble had picked out CGMs a few quasar-beams at a time, the new device let astronomers search around dozens of galaxies, using the light of even dimmer quasars.

    “It blew the field wide open,” Werk says.

    Gas flows out from Messier 82, the Cigar galaxy, to its invisible circumgalactic medium in this Hubble image. NASA, ESA, Hubble Heritage Team

    The circumgalactic census

    A team led by Jason Tumlinson of Baltimore’s Space Telescope Science Institute, Hubble’s academic home, made a catalog of 44 galaxies with a quasar sitting behind them from Hubble’s perspective. In a 2011 paper in Science, the researchers reported that every time they looked within 490,000 light-years of a galaxy, they saw spectra dappled with blank spots from atoms absorbing light. That meant that CGMs weren’t odd cloaks worn by just a few galaxies. They were everywhere.

    Tumlinson’s team spent the first few years after Hubble’s upgrade like 19th century naturalists describing new species. The group measured the mass and the chemical makeup of the galaxies’ CGMs and found they were huge cisterns of heavy elements. CGMs contain 10 million times the mass of the sun in oxygen alone. In many cases, the mass of a CGM is comparable to the mass of the entire visible part of its galaxy.

    The finding offers an answer to a long-standing cosmic mystery: How do galaxies have enough star-forming fuel to keep going for billions of years? Galaxies build stars from collapsing clouds of cool gas at a constant rate; the Milky Way, for example, makes one to two solar masses’ worth of stars every year. But there isn’t enough cool gas within the visible part of a galaxy, the disk containing its stars, to support observed rates of star formation.

    “We think that gas probably comes from the CGM,” Werk says. “But exactly how that gas is getting into galaxies, where it gets in, the timescale on which it gets in, are there things that prevent it from getting in? Those are big questions that keep us all awake at night.”

    Werk and Peeples realized that all that mass could help solve two other cosmic bookkeeping problems. All elements heavier than helium (which astronomers lump together as “metals”) are forged by nuclear fusion in the hearts of stars. When stars use up their fuel and explode as supernovas, they scatter those metals around to be folded into the next generation of stars.

    But if you add up all the metals in the stars, gas and dust in a given galaxy’s disk, it’s not enough to account for all the metals the galaxy has ever made. The mismatch gets even worse if you include the hydrogen, helium, electrons and protons — basically all the ordinary matter that should have collected in the galaxy since the Big Bang. Astronomers call all those bits baryons. Galaxies seem to be missing 70 to 95 percent of that stuff.

    So Peeples and Werk led a comprehensive effort to tally all the ordinary matter in about 40 galaxies observed with Hubble’s new spectrometer. The researchers published the results in two 2014 papers in The Astrophysical Journal.

    At the time, Werk reported that at least half of galaxies’ missing ordinary matter can be accounted for in their CGMs. In a 2017 update, Werk and colleagues found that the mass of baryons just in the form of cool gas in a galaxy’s CGM could be nearly 90 billion solar masses [The Astrophysical Journal]. “Obviously, this mass could resolve the galactic missing baryons problem,” the team wrote.

    “It’s a classic science story,” Schawinski says. The researchers had a hypothesis about where the missing material should be and made predictions. The group made observations to test those predictions and found what it sought.

    In a separate study, Peeples showed that although metals are born in galaxies’ starry disks, those metals don’t stay there. Only 20 to 25 percent of the metals a galaxy has ever produced remains in the stars, gas and dust in the disk, where the metals can be incorporated into new stars and planets. The rest probably ends up in the CGM.

    “If you look at all the metals the galaxies ever produced in their whole lifetime, more of them are outside the galaxy than are still inside the galaxy,” Tumlinson says, “which was a huge shock.”

    Recycling centers

    So how did the metals get into the CGM? Quasars’ spectra couldn’t help with that question. Their light shows only a slice through a single galaxy at a single moment in time. But astronomers can track galaxies’ growth and development with computer simulations based on physical rules for how stars and gas behave.

    This strategy revealed the churning, ever-changing nature of gas in galaxies’ CGMs. Simulations such as EAGLE, or Evolution and Assembly of GaLaxies and their Environments, which is run out of Leiden University in the Netherlands, showed that metals can reach CGMs through stars’ violent lives: in powerful winds of radiation blowing away from massive young stars, and in the death throes of supernovas spraying metals far and wide.

    This EAGLE simulation shows that, over time, metals (colors) move away from the center of a galaxy to the circumgalactic medium. J. Tumlinson, M.S. Peeples and J.K. Werk/Annual Review of Astronomy and Astrophysics 2017

    Once the metals are in the CGM, though, they don’t always stay put. In simulations, galaxies seem to use the same gas over and over again.

    “It’s basically just gravity,” Peeples says. “Throw a baseball up, and it’ll come back to the ground.” The same goes for gas flowing out of galaxies: Unless the gas travels fast enough to escape the galaxy’s gravity altogether, those atoms will eventually fall back into the disk — and form new stars.

    Some simulations show discrete gas parcels making the trip from a galaxy’s disk out into the CGM and back again several times. Together, CGMs and their galaxies are giant recycling devices.

    That means that the atoms that make up planets, plants and people may have taken several trips to circumgalactic space before becoming part of us. Over hundreds of millions of years, the atoms that eventually became part of you traveled hundreds of thousands of light-years.

    “This is my favorite thing,” Tumlinson says. “At some point, your carbon, your oxygen, your nitrogen, your iron was out in intergalactic space.”

    How galaxies die

    But not all galaxies get their CGM gas back. Losing the gas could shut off star formation in a galaxy for good. No one knows how star formation shuts off, or quenches. But the answer is probably in the CGM.

    Galaxies come in two main forms: young spiral galaxies that are making stars and old blobby galaxies where star formation is quenched (SN Online: 4/23/18).

    “How galaxies quench and why they stay that way is one of the most important questions in galaxy formation generally,” Tumlinson says. “It just has to have something to do with the gas supply.”

    Reading what’s not there

    Using light from a quasar (QSO), researchers can “see” CGMs. In this example, spectra from two galaxies, G1 and G2, have certain wavelengths missing (red, in bottom boxes) where the CGM atoms are absorbing light.


    One possibility, suggested in a paper posted online February 20 in The Astrophysical Journal, is that sprays of supernova-heated gas could get stripped from galaxies. Physicist Chad Bustard of the University of Wisconsin–Madison and colleagues simulated the Large Magellanic Cloud, a satellite galaxy of the Milky Way, and found that the small galaxy’s outflowing gas was swept away by the slight pressure of the galaxy’s movement around the Milky Way.

    Alternatively, a dead galaxy’s CGM gas could be too hot to sink into the galaxy and form stars. If so, star-forming galaxies should have CGMs full of cold gas, and dead galaxies should be shrouded in hot gas. Hot gas would stay floating above the galactic disk like a hot air balloon, too buoyant to sink in and form stars.

    But Hubble saw the opposite. Star-forming galaxies had CGMs chock-full of oxygen-VI — meaning that the gas was so hot (a million degrees Celsius or more) that oxygen atoms lost five of their original electrons. Dead galaxies had surprisingly little oxygen-VI.

    “That was puzzling,” Tumlinson says. “If theory told us anything, it should have gone the other way.”

    In 2016, Benjamin Oppenheimer, a computational astrophysicist at the University of Colorado Boulder, suggested a solution: The “dead” galaxies didn’t lack oxygen at all. The gas was just too hot for Hubble to observe. “In fact, there is even more oxygen around those passive galaxies,” Oppenheimer says.

    All that hot gas could potentially explain why those galaxies died — except that these galaxies were full of star-forming cold gas, too.

    “The dead galaxies have plenty of fuel left in the tank,” Tumlinson says. “We don’t know why they’re not using it. Everybody’s chasing that problem.”

    Grabbing at the elephant

    The chase comes at a good time. Until recently, observers had no way to map a single galaxy’s CGM. Researchers have had to add up dozens of quasar beams to understand the composition of CGMs on average.

    “We’ve been like the three blind people grabbing at the elephant,” says John O’Meara, an observational astronomer at Saint Michael’s College in Colchester, Vt.

    Teams using two new spectrographs — KCWI, the Keck Cosmic Web Imager on the Keck telescope in Hawaii, and MUSE, the Multi Unit Spectroscopic Explorer on the Very Large Telescope in Chile — are racing to change that.

    Keck Cosmic Web Imager schematic

    Keck Cosmic Web Imager

    Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft), above sea level,

    ESO MUSE on the VLT on Yepun (UT4),

    ESO VLT at Cerro Paranal in the Atacama Desert, •ANTU (UT1; The Sun ),
    •KUEYEN (UT2; The Moon ),
    •MELIPAL (UT3; The Southern Cross ), and
    •YEPUN (UT4; Venus – as evening star).
    elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo

    These instruments, called integral field spectrographs, can read spectra across a full galaxy all at once. Given enough background light, astronomers can now examine a single galaxy’s entire CGM. Finally, astronomers have a way to test theories of how gas circulates into and out of a galaxy.

    A Chilean team, led by astronomer Sebastian Lopez of the University of Chile in Santiago and colleagues, used MUSE to observe a small dim galaxy that happens to be sandwiched between a bright, distant galaxy and a massive galaxy cluster closer to Earth. The cluster acts as a gravitational lens, distorting the image of the distant galaxy into a long bright arc (SN: 3/10/12, p. 4). The light from that arc filtered through the CGM of the sandwiched galaxy, which the team called G1, at 56 different points.

    Surprisingly, G1’s CGM was lumpy, not smooth as expected, the team reported in the Feb. 22 Nature. “The assumption has been that that gas is distributed homogeneously around every system,” Lopez says. “This is not the case.”

    MUSE makes a mark

    Light from a source galaxy is deflected and magnified by an intervening galaxy cluster to form the bright arc seen in the projected image at far right. Unlike a quasar’s narrow beam of light, the extensive arc lights up a large area of galaxy G1’s CGM, showing it is surprisingly lumpy.


    O’Meara is leading a group that is hot on Lopez’s trail. Last year, while KCWI was being installed, O’Meara got an hour of observing time and was able to see hydrogen — which is associated with cool, star-forming gas — in the CGM of another galaxy backlit by a bright lensed arc. He’s not ready to discuss the results in detail yet, but the team is submitting a paper to Science.

    Meanwhile, Peeples’ team is revisiting how computers render CGMs. “The resolution of the circumgalactic medium in simulations is, um, bad,” she says. Existing simulations are good at matching the visible properties of galaxies — their stars, the gas between the stars, and the overall shapes and sizes. But they “utterly fail at reproducing the properties of the circumgalactic medium,” she says.

    So she’s running a new set of simulations called FOGGIE, which focus on CGMs for the first time. “We’re finding that it changes everything,” she says: The shape, star formation history and even the orientation of the galaxy in space look different.

    Together, the new observations and simulations suggest that the CGM’s function in the life cycle of a galaxy has been underestimated. Theorists like Peeples and observers like O’Meara are working together to make new predictions about how the CGM should look. Then the researchers will check real galaxies to see if they match.

    “Molly will post a really amazing new render of a simulation on Slack, and I’ll go, ‘Holy crap, that looks weird!’ ” O’Meara says. “I’ll go scampering off to find a similar example in the data, and we get into this positive feedback loop of going ‘Holy crap! Holy crap!’ ”

    While future circumgalactic studies will focus on gathering spectra from full CGMs, Tumlinson is hoping to squeeze more information out of Hubble while he still can. Hubble made CGM studies possible, but the telescope is 28 years old, and probably has less than a decade left. Hubble’s spectrograph is still the best at observing certain atoms in CGMs to help reveal the gaseous halos’ secrets. “It’s something we definitely want to do,” he says, “before Hubble ends up in the ocean.”

    See the full article here .


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  • richardmitnick 5:52 am on June 15, 2017 Permalink | Reply
    Tags: , , , , CGM - CircumGalactic Medium, , , It’s a gas gas gas: understanding gas motions surrounding galaxies, Keck Baryonic Structure Survey (KBSS)   

    From astrobites: “It’s a gas gas gas: understanding gas motions surrounding galaxies” 

    Astrobites bloc


    Jun 14, 2017
    Christopher Lovell

    Title: A comparison of observed and simulated absorption from HI, CIV, and SiIV around z ≈ 2 star-forming galaxies suggests redshift-space distortions are due to inflows
    Authors: M.L. Turner, J. Schaye, R. A. Crain, G. Rudie, C.C. Steidel, A. Strom, T. Theuns
    First Author’s Institution: MIT-Kavli Center for Astrophysics and Space Research, Massachusetts Institute of Technology
    Status: Submitted to the Monthly Notices of the Royal Astronomical Society, Open Access

    Figure 1: a schematic of the observational measurement. The light from the distant QSO (quasar) passes near to a galaxy and interacts with its associated gas. The absorption is dependent on many things: the position of the gas along the line of sight (LOS), the distance from the galaxy (Transverse Distance, TD), whether it’s infalling or outflowing, and even its rotation.

    For galaxies, gas is a pretty big deal. Without it, they’re unable to form new stars — which is pretty much their only job. Once they get hold of some though, stars of all shapes and sizes start forming. Some of these will be massive, rapidly burning up their fuel and going supernovae. If the supernovae is energetic enough, this can eject a load of precious gas and shut down the star formation again. Understanding this precarious galactic balancing act, between inflows and outflows of gas, is crucial for modelling the properties of galaxies.

    Flowing me, flowing you

    One way of picking apart this relationship is to look at where the inflows and outflows of gas meet. This area is approximately a few megaparsecs outside the galaxy, roughly 8 times the distance from the center of the galaxy as the edge of the disc of the Milky Way, and is known as the CircumGalactic Medium, or CGM. The gas in these regions doesn’t shine brightly of itself, so we have to infer its presence through other sources of light. One method takes advantage of Quasi-Stellar Objects (QSOs or quasars for short) behind the galaxy: as the light from a quasar passes through the CGM it gets absorbed by neutral hydrogen and metals (astronomer speak for any element heavier than helium) in the gas, and we can see this in its spectrum (see Figure 1). The pattern of absorption can tell us what metals are in the gas, and, importantly for this work, its direction of flow with respect to the galaxy.

    Today’s paper uses observations from the Keck Baryonic Structure Survey (KBSS), which studies the gas around 854 star forming galaxies at a redshift of two (around 3 billion years after the big bang) using background quasars. The authors compare the observations with mock spectra from the EAGLE simulation, a computer model of galaxy formation and evolution that matches various galaxy properties. The mock spectra are designed to mimic KBSS as closely as possible, so that comparisons can be made between the two. For this study they measure the optical depth (which roughly corresponds to the amount) of three elements: neutral hydrogen, ionised carbon and ionised silicon. Figure 2 shows two dimensional maps of these elements in both KBSS and the simulations – they look pretty similar!

    Figure 2: 2D maps of the amount of neutral hydrogen (HI), ionised carbon (CIV) and ionised silicon (SiIV) surrounding galaxies. The left column shows the observations, the right column shows the best fitting simulated galaxies. The bottom left corner of each panel corresponds to the position of the galaxy, and the x and y axes represent the transverse / line of sight distance from the galaxy, respectively.

    Tangled spectra

    Unfortunately, these measurements can get messy. Not only can the gas be inflowing or outflowing, but also rotating around the galaxy (see Figure 1). The host galaxy can also be moving with respect to the gas. All of this introduces uncertainties in the measured positions and velocities of the galaxy and its gas. The authors attempt to disentangle all of these effects, and find that the uncertainties on measurements of the galaxy distance (its redshift) have little effect: it’s the velocity of the gas that is important. They then try to pick apart its direction.

    In a simulation you know the motion of the gas directly, rather than having to infer it from a spectrum. The authors find that in EAGLE most of the gas is infalling, and since the mock spectra in the simulation are similar to the observations, the authors tentatively suggest that the gas in the observations could also be infalling. They also find that the more massive the dark matter halo hosting the galaxy, the higher the rate of infall.

    Dark matter halo Image credit: Virgo consortium / A. Amblard / ESA

    The optical depth is also insensitive to the amount of ‘feedback’ from supernovae in the simulation, supporting evidence for the idea that the higher observed gas densities are due to infalling, rather than outflowing, gas.

    Leaving on a simulation

    Today’s paper is a classic example of how simulations can help us understand our observations of the universe. By unpicking subtle features in the light of distant objects, which by chance happen to align with a galaxy, we can reveal the complicated relationships between galaxies and their surrounding gas. The authors also note that future studies of even greater detail could pick out other elements in the spectrum, such as Oxygen. These results are another step toward the ultimate goal of building a comprehensive model of galaxy evolution.

    See the full article here .

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